To be complete in our disclosure of the "ugliness", we must include the conquistadors, and the crusaders, and the inquisitors, and countless other butchers.
And while there is no disputing the existence of both theological and secular butchers throughout the course of history, it is not theology, nor is it secularity which is to blame.
It is a fundamental disregard for the rights of other human beings which carries the blame. And this is neither a uniquely theological OR secular trait.